Semiconductor lasers may be used in a wide variety of applications, such as for providing high-speed optical data links or for connecting workstations, peripherals and displays. In many applications, surface-emitting lasers provide a number of advantages over conventional edge-emitting lasers. The advantages include: (1) the devices are completed at a wafer level and therefore can be completely characterized; (2) the numerical aperture is smaller and symmetrical, allowing a highly efficient coupling to an optical fiber; (3) operation is single-frequency; and (4) the devices can be relatively easily integrated with monitor photodiodes or transistors, or may be integrated in two-dimensional arrays of surface-emitting lasers.
For a surface-emitting laser, radiation is typically emitted through one or more openings in a metal electrode of the laser. This arrangement is particularly advantageous if the light emitted from an opening is to be coupled to a single mode optical fiber. The shape and size of the opening through which light is coupled to the optical fiber can be made to conform to the shape and size of the fiber. On the other hand, light emitted from an edge-emitting laser is typically radiated from an elongated area, so that laser-to-fiber coupling is less efficient.
A surface-emitting laser is described in U.S. Pat. No. 5,266,503 to Wang et al., which is assigned to the assignee of the present invention. An opening in the electrode layer of Wang et al. is aligned with an electrically insulating region that may be formed by oxygen ion implantation. The region of implantation confines current flow from the electrode to a selected region of an active layer in which light energy is generated at a fixed wavelength in response to the current flow. The electrode and the current-confinement region may have a concentric arrangement.
Two concerns in the design and operation of a surface-emitting laser are the distribution of thermal energy and the distribution of current flow. These concerns are somewhat interrelated. A high thermal impedance will adversely affect performance of the laser. Current from the annular electrode is injected through a distributed Bragg reflector (DBR) mirror structure in which each layer has a thickness of one-quarter wavelength of the frequency of generated light propagating through the layers of the laser. In many cases, the one-quarter wavelength layers are too thin to handle the large current densities, e.g. 10-20 KA/cm.sup.2 injected into the mirror structure. This results in light output degradation and in a reduced useful life of the laser. A vertical cavity top-emitting laser is particularly susceptible to such problems. The uppermost layer is almost degenerately doped in order to reduce the contact resistance of the metal electrode. Since the layer is thin, it is susceptible to excessive heating that will result in burn-out.
U.S. Pat. No. 5,343,487 to Scott et al. describes a vertical cavity surface-emitting laser (VCSEL) that addresses the issue of current crowding, i.e. a non-uniform current distribution. In one embodiment, a contacting region is formed between an electrode and an active region of the VCSEL. The contacting region is provided with a layered section that is radially graded in resistivity, so as to restrict the current injection to a radius less than that of the resonant cavity of the laser. U.S. Pat. No. 5,245,622 to Jewell et al. describes a VCSEL that positions a stratified electrode between the active region of the laser and an upper DBR mirror structure. The stratified electrode comprises pairs of high doped and low doped layers having a conductivity type selected for injecting current into the active region of the laser. The thickness of the high doped layers is approximately equal to or less than one-quarter of the wavelength of emitted radiation. Likewise, the thickness of the low doped layers is approximately equal to or greater than one-quarter of the wavelength, with the total thickness of a pair being one-half wavelength. While use of the Scott et al. and Jewell et al. teachings provides improvements over previous laser structures, further improvements are desired.
What is needed is a surface-emitting laser that further improves current and/or heat distribution without adversely affecting the performance of the laser.